A gas turbine engine comprises, at the level of the turbine for example, seals between the outer radial ends of the rotor blades and the stator surface swept by the outer radial ends of the moving stage; it is a matter of preventing a portion of the driving gas not working by bypassing the turbine stage. Labyrinth seals are used for this purpose. These seals comprise a seal element integral with the rotor, formed from one or more strips, called wipers, arranged radially and transversely relative to the rotation axis of the machine. They also comprise a complementary stator element, facing the strips. This element has a surface portion that interacts with the strips to provide clearance that is as small as possible. Several strips in parallel cause a succession of pressure losses of the gas stream flowing round the turbine stage, providing the desired sealing.
These labyrinth seals are used in several places of the engine, for example also between the turbine shaft and the base of the stator stage, formed from stator vanes, between two successive moving stages of a turbine.
Depending on their operating conditions, gas turbine engines are subject to differential variations in expansion between in particular the elements of the stator and those of the rotor. If these differences in variation are not kept under control they may affect the clearances between the moving parts and the stationary parts. Thus, for an engine for propulsion of an aircraft undergoing flame extinction in the combustion chamber in flight for any reason, the whole turbine cools down because high-temperature gases are no longer passing through it. However, the casing may be cooled more quickly than the turbine rotor, which has an effect on the clearances. Now, for optimum efficiency, the clearances of the labyrinth seals are designed to be as small as possible in operation. In such a situation there is a consequent risk of blocking of the rotor owing to the more or less close contact of the wipers with the abradable material.
The characteristics of the labyrinth seal are determined and adjusted to allow a variation of clearance and possible slight contact during the different phases of flight, in normal operation, but when the dimensional variations are large, resulting from an incident of this kind, the labyrinth seal can no longer fulfil its role. As the rotor is prevented from turning, restarting of the engine by autorotation of the LP or HP body or else by being driven by an auxiliary engine may not proceed satisfactorily.
To prevent rotor lock-up in the case of such extinction of the combustion chamber, it would be conceivable to increase the clearance between the wipers and the abradable material. However, this solution is not economically viable owing to the drop in engine performance that it would cause.
The present applicant therefore pursued the aim of developing a solution that would make it possible to cope with the situation of untimely extinction of the combustion chamber without reducing the performance of the engine in normal operation.